Actually, although their lines may be angular and identical to each other, keeping in the current modern Lamborghini design, I don't really see how can a Gallardo be mistaken for a Murcielago, and likewise for the other way round.

The rear - if you compare them - they're completely different.
The side view - compare them - you'll see the Murcielago is much longer and looks more glued to the floor.
And there's the truck-like width of the Murcielago, of course.. the Gallardo is compact.